Thallus epilithic, +/- thin, continuous, sometimes cracked.

Apothecia not immersed in the rock; large to 0.5 mm diam.; at leat finally +/- sessile; smooth, distinct margin; carneous to brownish,  pigment K-; disc carneous (rarely chestnut brown); hymenium colourless or pale brownish to olivaceous in upper parts, I+ green-blue to red-brown.  Spores 7-12(15) X 5-7 µm; on wet acidic rocks near rivers, etc.  

This is the most difficult group because there is little variation in spore-size and pigmentation.  Extreme forms are therefore difficult to identify and obscure the understanding of the taxa.

I. arctica is a good example of these difficulties.  Some extreme specimens from the upper geolitteral are more areolate than usual and approaches I. rhodopsis.  There is, however, such a strong correlation between general morphology, spore size, ecology and distribution, that I have no doubts about accepting it as a distinct species, confined to the inundation zone of rivers and lakes in northern Scandinavia.

Magnusson (op. cit.) recognized many varieties of this species [I. euplotica].  I recognize var. arctica as a separate species, and the rest are included in the variable I. rhodopsis.  Mixed collections of I. rhodopsis and I. euplotica have been observed, indicating that they are genetically distinct and should be treated in the same way as I. heteromorpha/I.melanocarpa.